This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Molybdenum-containing enzymes are widespread, catalyzing atom transfer reactions coupled to intramolecular electron transfer (IET). Sulfite oxidase is the prototypical member of one major family of Mo enzymes, and carries out IET during turnover with a rate of ~2000 s^-1. With the sole exception of sulfite oxidase, all Mo enzyme crystal structures show separations of electron donor and acceptor sites well within the expected range for IET. For sulfite oxidase, however, crystal structures show large separations of ~32 [unreadable] between the Mo donor site in the C-terminal domain and the electron-accepting group in the N-terminal domain. Such large separation would appear to preclude IET at the rates observed. Docking of the N-terminal domain with the larger C-terminal domain (a more compact conformation) has been proposed to facilitate IET however there is no experimental evidence for this. The proposed study seeks to provide such evidence.